Synthetic sonar antenna

ABSTRACT

The invention relates to a synthetic sonar antenna and more particularly to the self-calibration of such an antenna. It consists in minimizing the number of sensors of the reception antenna by fixing the reduced pitch between the sensors, imposed by the precision of the self-calibration, solely at the two ends of the antenna. According to a variant, the reduced pitch is fixed at just one end of the antenna. It makes it possible to increase the precision of the self-calibration with a smaller number of sensors than in the prior art.

The present invention pertains to sonar antennas which are used to forma synthetic antenna and more particularly to the self-calibration ofsuch a synthetic antenna.

Synthetic antennas are well known, both in the field of radar and thatof sonar, and the self-calibration of such antennas is a technique knownper se, described in particular in an article by Didier Billon andFranck Fohanno published in the proceedings of “OCEAN 98” by IEE onpages 965 to 970.

The self-calibration of a synthetic antenna based on inter-recurrencecorrelation consists in estimating a length L of a displacement parallelto the antenna, a delay τ between two correlated signals, and avariation in bearing β induced by the rotation of the antenna, fromsignals of two successive recurrences, received over two time intervalscorresponding to one and the same distance interval and small enough forit to be possible to assume that these parameters are constant. Thelength L is the sum, on the one hand, of the lengths of the componentsparallel to the antenna of the displacement between the two emissioninstants and on the other hand, of the displacement between tworeception instants relating to the center of the distance intervalconsidered. If Oξ is the antenna axis oriented in the direction of thedisplacement, the self-calibration is based on the correlation betweenthe signal received at the point with abscissa ξ at the first recurrenceand at the point with abscissa ξ−L at the second recurrence.

The precision of the estimates depends in particular on the number ofpairs (ξ,ξ−L) of abscissas along the antenna such that the incidentacoustic field generated by the reverberation of the sea bed hasindependent values at the abscissas ξ lying between ξ₁+L and ξ_(M), ξ₁and ξ_(M) being the abscissas of the phase centers of the extremesensors of the antenna.

The number of independent values of the acoustic field along thereception antenna is equal to the ratio of the length of the antennaL_(r) to the correlation length of the field along the antenna, which isequal to the ratio λ/Δθ of the wavelength λ and of the bearing width Δθof the emission sector.

In the known art, the pitch between the sensors of the reception antennamust be less than this field correlation length, the ratio between thetwo lengths having in practice a value of the order of 1.5. Statedotherwise, the number of sensors must be greater than the number ofindependent values of the incident field along the antenna in the sameratio.

Thus, increasing the number of independent values of the field along theantenna involves, in the known art, reducing the pitch between sensors,hence increasing the number thereof. The invention aims to obtain thesame effect with a lesser increase than in the prior art.

The invention proposes, in its preferred embodiment, a synthetic sonarantenna, characterized in that the geometry of the reception antenna isoptimized in such a way as to minimize the number of sensors by fixingthe pitch imposed by the precision of the self-calibration at at leastone end of the reception antenna.

Other features and advantages of the invention will become clearlyapparent in the following description, presented by way of nonlimitingexample with regard to the appended figures which represent:

FIG. 1, the diagram of the reception antenna according to the preferredembodiment of the invention and,

FIG. 2, the diagram of this antenna according to a variant embodiment ofthe invention.

In the subsequent description, the term “antenna” on its own relates toa reception antenna.

To maximize the degree of temporal coverage of the sonar system, it isnecessary for the speed of the platform to be close to the upper limitimposed by the principle of the synthetic antenna. In this case, thelength L/2 of the inter-recurrence displacement measured parallel to theantenna is close to the length L_(r) of the antenna. Thus the ends withwhich the inter-recurrence correlation is done, of length L_(r)−L, thencomprise an insufficient number of sensors to allow satisfactory spatialinterpolation.

The invention proposes, as represented in FIG. 1, to reduce the pitch atthe two ends of the antenna 101, the remainder of the antenna retaininga pitch which is determined, as in the known art, by the desired levelof the image lobe in the directivity pattern of a channel, which inpractice leads to fixing a pitch for example of the order of 0.7 λ/Δθdenoted d.

According to a variant, the pitch is reduced at just one end.

The reduced pitch being denoted d′, let N be the number of sensors withthe nominal pitch of d and N′ the number of sensors with the reducedpitch d′ (see FIG. 1). The total number of sensors is then M=N+N′. Inorder for it to be possible to determine by interpolation at least onepair of signals (s(ξ, t), s(ξ−L, t+T_(r)))consisting of a signal of afirst recurrence and of a signal of a second recurrence whose phasecenters are L apart, the parameter of the self-calibration being definedabove, it is necessary to satisfy the inequality:L≦ξ _(M)−ξ₁  (1)

The length of the antenna being fixed and given by the expressionL_(r)=Nd+N′d′, relation (1) may be written in the equivalent manner:L≦L _(r) −d  (2)if the pitch is constant therein as in the known art (N′=0), or else,$\begin{matrix}{L \leq {L_{r} - \frac{d + d^{\prime}}{2}}} & \left( 2^{\prime} \right)\end{matrix}$if the pitch is reduced at only one of the two ends, or else again,L≦L−d′  (2″)if the pitch is reduced at both ends.

Thus, the length L_(r) being fixed, the limit on L is larger than in theknown art if the pitch is reduced at one end, and larger again if thepitch is reduced at both ends. This advantage is acquired provided thatat one end with reduced pitch, there is at least one sensor.

When d is much less than L_(r), the lesser limitation of L, hence of thespeed of the platform, constitutes only a small relative advantage withrespect to the known art. However, this analysis takes no account of thequality required by the spatial interpolation of the self-calibration ofthe synthetic antenna.

Depending on the resolution gain desired for the synthetic antennaprocessing, the pitch d is determined conventionally as a function ofcriteria of quality of directivity of the channels formed. Now, itsvalue generally turns out to be incompatible with the necessaryself-calibration precision.

The method of the invention allows a substantial saving in this case inthe number of sensors of the antenna, in a ratio close to that of thepitch d determined conventionally and of the pitch d′ fixed by theprecision of the self-calibration if the antenna ends carrying theinter-recurrence correlation have a much smaller length than the totallength of the antenna. This case may in particular arise when utilizingthe self-calibration method described in the patent applicationpublished in France under No. 2 769 372. Disclosed therein is a methodmaking it possible to reduce the length of these ends while preservingsufficient self-calibration precision by virtue of the inclusion in theprocessing of gyrometric measurements.

Condition (1) is valid only if a linear interpolation is sufficient. Forexample, if ξ₁<ξm−L<ξ₂, the interpolation is done according to theformula: $\begin{matrix}{{s\left( {{\xi_{M}\quad L},{t + T_{r}}} \right)} = \frac{{\left( {{\xi_{2}\quad\xi_{M}} + L} \right)\quad s\quad\left( {\xi_{1},{t + T_{r}}} \right)} + {\left( {\xi_{M}\quad L\quad\xi_{1}} \right)\quad s\quad\left( {\xi_{2},{t + T_{r}}} \right)}}{\xi_{2}\quad\xi_{1}}} & (3)\end{matrix}$This interpolation may turn out to be insufficiently precise in the caseof an antenna whose constant pitch d is determined, as in the prior art,independently of the constraint related to the precision of theinterpolation in the self-calibration. If K is the order of theinterpolation required with such an antenna, K=1 corresponding to alinear interpolation, K=2 corresponding to a parabolic interpolation andso on and so forth, it is then necessary to replace if K>2, condition(2) with the stricter condition L≦L_(r) (K+1)d/2.

The reduced pitch d′ may be chosen so that the linear interpolation (3)is sufficient. It then suffices for there to be two sensors with reducedpitch at one of the two ends. Specifically, instead of interpolating thesignals, received at t+T_(r), of the first two sensors of the secondrecurrence, to determine the signal with phase center ξ_(M)−L, thesignals, received at t, of the last two sensors of the first recurrenceare interpolated according to a formula analogous to (3) to determinethe signal with phase center ξ₁+L.

Generally, the implementation of the method of the invention does notinvolve reducing the pitch over the entire length L_(r)−L of one end ofthe antenna, the benefit of the reduced pitch with respect to thenominal pitch being real only for the few end sensors, typically forexample between 1 and 4, so as to alleviate the loss of precision of thespatial interpolation due to the edge effects.

1. A synthetic sonar antenna, the geometry of the reception antennabeing optimized in such a way as to minimize the number of sensors byfixing the pitch imposed by the precision of the self-calibration at atleast one end of the reception antenna.
 2. The antenna as claimed inclaim 1, wherein in that the number of sensors is increased at each endof the antenna with respect to the central part so as to obtain a finerpitch of these sensors at these ends.
 3. The antenna as claimed in claim1, wherein in that the number of sensors is increased at just one end ofthe antenna with respect to the central part so as to obtain a finerpitch of these sensors at this end.